The differentiation of enzyme expression in normal and neoplastic mammary glands and bone marrow cells is being studied in order to define those processes which regulate the appearance or loss of activity and, thus, modify cellular function. One specific case currently under investigation is the mechanisms by which polyamines participate in overall cellular metabolism; another is the effect of hormones on cellular reactions during and after neoplastic transformation. We are preparing to study the effects of mammary tumor virus infection on the concentration of cyclic AMP and other regulatory enzymes in mammary glands of susceptible and resistant inbred strains of mice.